I know it's only a stop-gap for this tour, but to be honest I think Coquelin would make a great CB.
Although he's not tall, his tackling and anticipation is unbelievable - if he can learn the craft of the CB I think he'd be amazing there.
Hearing that Xhaka is good in the air. That and someone like Giroud might be able to make up for the lack of height when defending set pieces hopefully. But aside from that weakness, there's a lot to like about the idea. Not to mention we're now adequately covered at DM, with Xhaka and Elneny both purchased since Coquelin famously came back from loan.